This invention relates in general to vehicle disc brake assemblies and in particular to an improved structure for a brake shoe retractor adapted for use in such a vehicle disc brake assembly.
Most vehicles are equipped with a brake system for retarding or stopping movement of the vehicle in a controlled manner. A typical brake system for an automobile or light truck includes a disc brake assembly for each of the front wheels and either a drum brake assembly or a disc brake assembly for each of the rear wheels. The brake assemblies are typically actuated by hydraulic or pneumatic pressure generated when an operator of the vehicle depresses a brake pedal. The structures of these drum brake assemblies and disc brake assemblies, as well as the actuators therefor, are well known in the art.
A typical disc brake assembly includes an anchor plate which is secured to a fixed, non-rotatable component of the vehicle. A pair of brake shoes are supported on the anchor plate for sliding movement relative thereto. The brake shoes have respective friction pads which are disposed on opposite sides of a rotor. The rotor, in turn, is connected to the wheel of the vehicle for rotation therewith. To effect braking action, the brake shoes are moved inwardly toward one another so as to frictionally engage the opposed sides of the rotor. Such frictional engagement causes retarding or stopping of the rotational movement of the rotor and, therefore, the wheel of the vehicle in a controlled manner.
To accomplish this, the disc brake assembly further includes a caliper assembly for selectively moving the brake shoes into frictional engagement with the rotor. The caliper assembly includes a pair of pins having outer ends which are secured to the anchor plate. The inner ends of the pins extend inwardly away from the anchor plate and the vehicle wheel. A caliper is supported on the pins for sliding movement relative to the anchor plate. The caliper is generally C-shaped, having an inboard leg disposed adjacent the inboard brake shoe and an outboard leg disposed adjacent to the outboard brake shoe. One or more hydraulically or pneumatically actuated pistons are provided in respective cylindrical recesses formed in the inboard leg of the caliper adjacent to the inboard brake shoe. When the brake pedal is depressed, the piston and the inboard leg of the caliper are urged apart from one another. Specifically, the piston is urged outwardly, while the inboard leg of the caliper is urged inwardly. As mentioned above, the piston is disposed adjacent to the inboard brake shoe and, therefore, urges it outwardly toward the inner side of the rotor. Because the caliper is slidably mounted on the pins of the anchor plate, the outboard leg of the caliper (and, thus, the outboard brake shoe disposed adjacent thereto) are urged inwardly toward the outer side of the rotor. As result, the brake shoes frictionally engage the opposed sides of the rotor.
Frequently, an annular roll-back seal is provided within the cylindrical recess in contact with the outer surface of the piston. The roll-back seal is conventional in the art and performs several functions. First, the roll-back seal provides a seal to define the extent of the cylindrical recess within which the piston is disposed. Second, the roll-back seal is designed to retract the piston inwardly away from the rotor by a predetermined distance from the fully engaged position when the brake pedal is released after being depressed. To accomplish this, the roll-back seal frictionally engages the outer surface of the piston, resiliently resisting movement thereof when the brake pedal is depressed. Thus, when the brake pedal is released by the operator of the vehicle, the resilience of the roll-back seal causes the piston to retract within the cylindrical recess and out of contact with the inboard brake shoe.
It has been found with repeated usage, the friction pads of the brake shoes wear and become increasingly thinner. When this occurs, the piston and the caliper must move greater distances relative to one another to effect the same braking action as when the friction pads were new. Despite this increased distance of movement of the piston in the outboard direction, it is desirable that the roll-back seal retract the piston only by the same predetermined distance away from the rotor in the inboard direction. Thus, known roll-back seals are designed to accommodate increased movement of the piston in the outboard direction when the brake pedal is depressed, but to retract the piston inwardly by approximately the same predetermined distance when the brake pedal is subsequently released.
Ideally, when the brake pedal is released, the brake shoes should also be spread apart from one another to prevent any incidental frictional engagement with the rotor. To accomplish this, it is known to provide the disc brake assembly with one or more retraction springs for moving the brake shoes apart from one another when the brake pedal is released. For example, U.S. Pat. Nos. 4,364,455 to Oshima, 4,491,204 to Dirauf et al., 4,629,037 to Madzgalla et al., 4,658,938 to Thiel et al., 4,867,280 to Von Gruenberg et al., 4,940,119 to Kondo et al., 5,069,313 to Kato et al., 5,249,647 to Kobayashi et al., and 5,251,727 to Loeffier et al. all disclose disc brake assemblies which include a retraction spring structure. It is desirable that such retraction springs exert an amount of force which is large enough to urge the brake shoes apart from one another, but small enough not to overcome the roll-back seal to move the piston deeper within the associated cylindrical recess formed in the inboard leg of the caliper.
Unfortunately, when the friction pads of the brake shoes wear thinner as described above, conventional retraction springs are compressed to a greater extent when the friction pads of the brake shoes are worn than when they are new. The additional compression of known retraction springs can result in the exertion of a force against the brake shoe which can overcome the roll-back seal and undesirably move the piston away from the rotor by a distance which is greater than the predetermined distance discussed above. Thus, it would be desirable to provide an improved retraction mechanism for use in a disc brake assembly which applies a generally constant restoring force urging the brake shoes apart from one another regardless of the amount of wear of the friction pads.